In order to realize semiconductor devices having excellent characteristics, such as a thin film transistor, a thin film diode, and a solar cell, it is essential for a semiconductor film used as an active layer to have a high carrier (electron or hole) mobility.
A semiconductor film whose mobility is high, for example, a semiconductor film made of polycrystalline Si, is normally formed by a high-temperature process. Therefore, there is a problem in that a low-cost substrate such as a resin substrate which is exemplified by a low-softening point glass substrate or a plastic substrate cannot be used.
In order to solve this problem, there is developed a technology called a low-temperature process, for irradiating an amorphous Si film with a high-power pulse laser beam to form a polycrystalline Si film, so a relatively low-cost heat-resistant glass can be used. However, the stability of the laser beam is not sufficient, so it is difficult to uniformly process the entire surface of a large-area substrate. Therefore, it is difficult to obtain multiple semiconductor devices having uniform characteristics on the same substrate. There is also a problem in that a throughput is low because an irradiation area of the laser beam is small.
In recent years, an oxide semiconductor thin film which is made of, for example, Zn—O, In—Ga—Zn—O, or In—O and can be formed at low temperature without laser beam irradiation has been under active development.
It has been known that the hole mobility of the oxide semiconductor thin film formed even at room temperature is a relatively high value equal to or larger than 10 cm2/Vs. There have been made attempts to form a flexible thin film transistor (TFT) on a flexible substrate such as a plastic substrate or a plastic film.
For example, a technology for a TFT using as the active layer an oxide film containing Zn—O as a main component is described in Japanese Patent Application Laid-Open No. 2002-76356.
A technology for a TFT using as the active layer an amorphous oxide film which is formed at room temperature and contains In, Zn, and Ga is described in Nature, Vol. 432, 25, Nov. 2004 (488-492).
A TFT using as the active layer an oxide thin film which is formed at room temperature and contains In—O as a main component is described in Nature Materials, Vol. 5, November 2006 (893-900).
In the case of the oxide semiconductor described in Nature, Vol. 432, 25, Nov. 2004 (488-492), the hole mobility value thereof at room temperature is in a range of 5 cm2/Vs to 10 cm2/Vs. The field effect mobility value of the TFT using the oxide semiconductor for active layer is in a range of 6 cm2/Vs to 9 cm2/Vs. Therefore, there have been expected applications to an active matrix desired for a flat panel display using a liquid crystal device or an electroluminescence device. However, when the above-mentioned material is used, it is difficult to further increase the mobility. Thus, there is a limitation on high-speed operation, so its application is limited.
In contrast, in the case of the oxide semiconductor described in Nature Materials, Vol. 5, November 2006 (893-900), the hole mobility value thereof at room temperature is approximately 30 cm2/Vs. The field effect mobility value of the TFT using the oxide semiconductor for an active layer is in a range of 10 cm2/Vs to 140 cm2/Vs depending on the gate insulating film material.
According to the studies made by the present inventors, it is found that an In—O film formed at room temperature has low environmental stability and the resistivity thereof significantly changes when the In—O film is left in the atmosphere. For example, when the In—O film is left in the atmosphere at a temperature of 20° C. and a humidity of 50% for a month, a reduction of one or two orders of magnitude in resistivity is observed. The aforementioned reduction in resistivity is observed even in the case of the oxide semiconductor containing Zn—O as a main component as described in Japanese Patent Application Laid-Open No. 2002-76356.
In the case of the oxide semiconductor described in each of Japanese Patent Application Laid-Open No. 2002-76356, Nature, Vol. 432, 25, Nov. 2004 (488-492), and Nature Materials, Vol. 5, November 2006 (893-900), the band gap thereof is a large value of approximately 3 eV in terms of optical characteristics. Therefore, it is difficult to say that the oxide semiconductor is suitable for a light receiving device for which photosensitivity is required in a visible light region, such as a visible light receiving device or a solar cell. Thus, its application fields are limited.